Completing Memory Sequence 9 in under 7 minutes is quite easy. It can be done in under 6 minutes for the "increased Smoke Ammo" upgrade. You can save a lot of time by jumping instead of running. The hardest part is the Boss fight at the end. Sometimes the Boss runs away and hides in the crowd, which can cost you a lot of time. The best way to deal with this is by restarting the checkpoint when it takes too long. Additionally, cutscenes do not add to your time. There is also no penalty for restarting checkpoints.

Easy "Faster Than Light" achievement

Completing Memory Sequence 4 in under 9 minutes is not too hard. You can complete the sequence in approximately 8 minutes -- so there is some room for error. You should run at all times (do not walk slowly). In the climbing sections, press X to move quicker. The fastest way to kill enemies is to throw a smoke bomb, then perform an assassination while sliding under them. If you took too long during a section, you can restart the checkpoint. Restarting checkpoints does not add to your time.

Easy "Gone In A Flash" achievement

The best farming spot to do this is in Memory Sequence 7 in New Game Plus mode (second playthrough). After reaching the first checkpoint, there will be four enemies coming out a door. Jump in front of the door before they spawn and press Trianlge to blend in with the background and become invisible. Let the enemies walk past you and restart the checkpoint. You must pick up the purple helix boosts before the first checkpoint. Helix Blend is unlocked in Sequence 8 on the first playthrough. This means it will not be available on the Normal difficulty during Memory Sequence 7. You must be playing New Game Plus mode for this farming method to work.

Easy "Sharp Shooter" achievement

This can be done at the very end of Memory Sequence 8. After climbing a tower, some enemies shoot at you with big cannons (mortars) from the ground. You will automatically pick up a sniper rifle and have to kill the mortar enemies. To get the "Sharp Shooter" achievement, you must quickly kill them before they can fire at you. There are five enemies. Activate Eagle Vision to see them more easily. Just wait until they reach the mortar, then they will stand still for a moment and you can easily kill them.
You can restart the checkpoint if you have taken damage from the mortars.